Micro-alloyed porous metal having optimized chemical composition and method of manufacturing the same

ABSTRACT

A micro-alloyed porous metal is disclosed having an optimized chemical composition to achieve targeted mechanical properties for use as an orthopaedic implant and a cell/soft tissue receptor. The porous metal may achieve a targeted compressive strength and a targeted ductility, for example. These targeted mechanical properties may allow the porous metal to be densified to a low relative density.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional PatentApplication Ser. No. 61/497,780, filed Jun. 16, 2011, the disclosure ofwhich is hereby expressly incorporated by reference herein in itsentirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to a porous metal for use as anorthopaedic implant. More particularly, the present disclosure relatesto a micro-alloyed porous metal having an optimized chemical compositionto achieve targeted mechanical properties for use as an orthopaedicimplant, and to a method for manufacturing the same.

BACKGROUND OF THE DISCLOSURE

Orthopaedic implants may be constructed of porous metal to encouragebone growth into the orthopaedic implant. An example of such a materialis produced using Trabecular Metal™ technology generally available fromZimmer, Inc., of Warsaw, Ind. Trabecular Metal™ is a trademark ofZimmer, Inc. Such a material may be formed from a reticulated vitreouscarbon (RVC) foam substrate which is infiltrated and coated with abiocompatible metal in the manner disclosed in detail in U.S. Pat. No.5,282,861 to Kaplan, the entire disclosure of which is expresslyincorporated herein by reference. The resulting infiltrated and coatedmaterial is lightweight, strong, and has open cells that are similar tothe structure of natural cancellous bone, thereby providing a matrixinto which cancellous bone may grow to fix the orthopaedic implant tothe patient's bone. The coated metal layer of the material may containup to 2,000 ppm oxygen, up to 2,000 ppm nitrogen, and up to 500 ppmhydrogen. However, to achieve desired mechanical properties with thiscoated metal layer, the material is densified to a relative density of18% or more, such as from 18% to 25%.

SUMMARY

The present disclosure relates to a micro-alloyed porous metal having anoptimized chemical composition to achieve targeted mechanical propertiesfor use as an orthopaedic implant and a cell/soft tissue receptor, andto a method for manufacturing the same. The porous metal may achieve atargeted compressive strength (e.g., 24,000 psi or more) and a targetedductility (e.g., 50% or more), for example. These targeted mechanicalproperties may allow the porous metal to be densified to a lowerrelative density than is currently manufactured commercially. Forexample, the porous metal may be densified to a relative density lessthan 18%.

According to an embodiment of the present disclosure, a highly porousbiomaterial is provided that is configured to be implanted in apatient's body. The highly porous biomaterial includes a poroussubstrate having a plurality of ligaments that define pores of theporous substrate and a biocompatible metal coating applied to theplurality of ligaments of the porous substrate, the highly porousbiomaterial having a relative density less than 18%, the relativedensity being a percentage obtained by dividing an actual density of thehighly porous biomaterial by a theoretical density of the biocompatiblemetal of the coating.

According to another embodiment of the present disclosure, a method isprovided for manufacturing a highly porous biomaterial. The methodincludes the steps of: providing a porous substrate having a pluralityof ligaments that define pores of the porous substrate; depositing abiocompatible metal coating onto the plurality of ligaments of theporous substrate; and setting at least one of a maximum oxygenconcentration in the metal coating at 1,212 ppm, and a maximum nitrogenconcentration in the metal coating at 1,243 ppm.

According to yet another embodiment of the present disclosure, a methodis provided for manufacturing a highly porous biomaterial. The methodincludes the steps of: providing a porous substrate having a pluralityof ligaments that define pores of the porous substrate; depositing abiocompatible metal coating onto the plurality of ligaments of theporous substrate; and setting a minimum nitrogen concentration in themetal coating at 488 ppm.

According to yet another embodiment of the present disclosure, a methodis provided for manufacturing a highly porous biomaterial. The methodincludes the steps of: providing a porous substrate having a pluralityof ligaments that define pores of the porous substrate; and depositing abiocompatible metal coating onto the plurality of ligaments of theporous substrate to a completed extent, the highly porous biomaterialhaving a relative density less than 18% at the completed extent, therelative density being a percentage obtained by dividing an actualdensity of the highly porous biomaterial by a theoretical density of thebiocompatible metal of the coating.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of thisdisclosure, and the manner of attaining them, will become more apparentand the invention itself will be better understood by reference to thefollowing description of embodiments of the invention taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is a flow diagram of an exemplary method of the presentdisclosure;

FIG. 2 is a perspective view of an orthopaedic implant manufacturedaccording to the method of FIG. 1, the orthopaedic implant being formedof a highly porous material;

FIG. 3 is a schematic diagram of a chemical vapor deposition apparatusused to perform the method of FIG. 1;

FIG. 4A is an experimental graphical representation of the specificcompressive strength of the highly porous material based on theconcentration of oxygen in the material;

FIG. 4B is an experimental graphical representation of the specificcompressive strength of the highly porous material based on theconcentration of nitrogen in the material;

FIG. 5A is an experimental graphical representation of the ductility ofthe highly porous material based on the concentration of oxygen in thematerial; and

FIG. 5B is an experimental graphical representation of the ductility ofthe highly porous material based on the concentration of nitrogen in thematerial.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate exemplary embodiments of the invention and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION

FIG. 1 provides an exemplary method 100 for manufacturing amicro-alloyed porous metal having an optimized chemical composition toachieve targeted mechanical properties for use as an orthopaedic implantand a cell/soft tissue receptor.

Beginning at step 102 of method 100 (FIG. 1), a porous lattice orsubstrate is provided having a large plurality of ligaments that defineopen-cells or pores therebetween. An exemplary porous substrate is a RVCfoam substrate having a large plurality of vitreous carbon ligamentsthat define dodecahedron (12-sided) pores therebetween. RVC foam iscommercially available in porosities ranging from 10 to 200 pores perinch, and more specifically in porosities of 65, 80, and 100 pores perinch. Such RVC foam substrates may be formed by pyrolyzing an open-cell,polymer foam. During step 102 of method 100, the RVC foam substrate mayhave a bulk shape (e.g., a block), a near-net shape (e.g., a solidhemisphere), or a net shape (e.g., a hollow hemisphere), for example.

Continuing to step 104 of method 100 (FIG. 1), the ligaments of theporous substrate are coated with a thin film of biocompatible metal.With reference to FIG. 2, for example, the vitreous carbon ligaments 206of the porous substrate are coated with a thin film of biocompatiblemetal 208. In this manner, the underlying porous substrate serves as askeleton for the biocompatible metal coating.

In an exemplary embodiment of the present disclosure, tantalum or analloy thereof is used to coat the porous substrate during the coatingstep 104 of method 100 (FIG. 1). Other suitable biocompatible metalsthat may be used to coat the porous substrate include other refractory(Group IV-VI) metals, such as titanium, niobium, hafnium, tungsten, andalloys thereof, for example. Such refractory metals generally retaintheir mechanical strength at high temperatures and have a high affinityfor interstitial elements, including oxygen.

Also in an exemplary embodiment of the present disclosure, a chemicalvapor deposition (CVD) process is performed to coat the porous substrateduring the coating step 104 of method 100 (FIG. 1). An exemplary CVDprocess is described in the above-incorporated U.S. Pat. No. 5,282,861to Kaplan.

With reference to FIG. 3, apparatus 300 is provided to perform the CVDprocess. FIG. 3 is schematic in nature, and it is understood that thedesign of apparatus 300 may vary. Apparatus 300 includes housing 302that defines an internal reaction chamber 304. Apparatus 300 includes achlorine (Cl₂) gas input 310, a hydrogen (H₂) gas input 312, and an airinput 314 into reaction chamber 304, each having a suitable flow controlvalve (not shown). Apparatus 300 also includes an exhaust gas output 316from reaction chamber 304. Within reaction chamber 304, apparatus 300includes a heated chlorination chamber 320 and a heated depositionchamber or furnace 322. A supply of tantalum 330 or anotherbiocompatible metal is located within chlorination chamber 320, and aporous substrate 332 is located within deposition chamber 322.

In operation, Cl₂ gas is injected via input 310 and H₂ gas is injectedvia input 312 into reaction chamber 304, which may be held under vacuumat a pressure of 1.0 to 2.0 Torr. Once inside the heated chlorinationchamber 320, which may be resistance-heated to a temperature ofapproximately 500° C., the Cl₂ gas reacts with tantalum 330 to formtantalum chloride gas, such as TaCl₅ gas. The TaCl₅ gas then mixes withthe injected H₂ gas and travels into the heated deposition chamber 322,which may be induction-heated to a temperature of approximately 900°C.-1,100° C., and more specifically to a temperature of approximately900° C.-970° C. Once inside the heated deposition chamber 322, the TaCl₅and H₂ gases flow around and into the porous substrate 332. Then, uponcontact with the heated surfaces of porous substrate 332, the TaCl₅ andH₂ gases react to deposit tantalum metal and to liberate hydrogenchloride (HCl) gas. As shown in FIG. 2, the liberated tantalum metal isdeposited as a thin, substantially uniform film 208 onto exterior andinterior vitreous carbon ligaments 206 of the porous substrate. The HClgas is then exhausted via exhaust gas output 316 from reaction chamber304, along with excess reactant gases.

To promote even metal deposition and infiltration, the porous substrate332 may be flipped and/or rotated in apparatus 300 during the CVDprocess or between individual cycles of the CVD process. Also, poroussubstrate 332 may be moved to different locations in apparatus 300,especially when multiple porous substrates 332 are coated simultaneouslyin apparatus 300. For example, when apparatus 300 contains a stack ofporous substrates 332, a certain substrate may be located on top of thestack during a first CVD cycle and then may be moved to the bottom ofthe stack during a second CVD cycle.

Returning to FIG. 2, the above-described CVD process producesorthopaedic implant 200 having a large plurality of ligaments 202 thatdefine open-cells or pores 204 therebetween, with each ligament 202including a vitreous carbon core 206 covered by a thin film of depositedmetal 208. Orthopaedic implant 200 is a highly porous structure having aporosity as low as 55%, 65%, or 75% and as high as 80% or 85%. Theopen-cells or pores 204 between ligaments 202 of orthopaedic implant 200form a matrix of continuous channels having no dead ends, such thatgrowth of cancellous bone, cells, and soft tissue through the structureis uninhibited. The highly porous structure is also lightweight, strong,and substantially uniform and consistent in composition.

The highly porous structure may be made in a variety of densities inorder to selectively tailor orthopaedic implant 200 for particularapplications. In particular, as discussed in the above-incorporated U.S.Pat. No. 5,282,861 to Kaplan, the highly porous structure may befabricated to virtually any desired porosity and pore size, and can thusbe matched with the surrounding natural bone in order to provide anoptimized matrix for bone ingrowth and mineralization.

To achieve targeted mechanical properties, specifically a targetedcompressive strength and a targeted ductility, the deposited metal film208 on orthopaedic implant 200 may be micro-alloyed with controlledamounts of certain interstitial elements. In certain embodiments, thedeposited metal film 208 on orthopaedic implant 200 may be micro-alloyedwith controlled amounts of nitrogen, oxygen, and/or hydrogen. Suchmicro-alloying may occur during the above-described CVD process bycontrolling the relative amounts of Cl₂ gas delivered via input 310, H₂gas delivered via input 312, and air delivered via input 314 (FIG. 3).Suitable gas flow rates are set forth in Table 1 below.

TABLE 1 Flow Rate Range Input Gas (sccm) Chlorine (Cl₂) 600-984 sccmHydrogen (H₂) 1150-2200 sccm Air Atmospheric Air (Nitrogen 10-40 sccm(N₂) and Oxygen (O₂)) or Pure or Substantially Pure N₂

According to an exemplary embodiment of the present disclosure, thedeposited metal film 208 on orthopaedic implant 200 is micro-alloyedaccording to Table 2 below. In this embodiment, the minimum nitrogenconcentration of 488 ppm may ensure that orthopaedic implant 200 hassufficient compressive strength, while the maximum oxygen concentrationof 1,212 ppm may ensure that orthopaedic implant 200 has sufficientductility. The balance may include primarily tantalum and other elementssuch as iron, tungsten, molybdenum, silicon, and nickel, for example.

TABLE 2 Minimum Maximum Concentration Concentration Element (ppm (w/v))(ppm (w/v)) Nitrogen 488 2,200 Oxygen 0 1,212 Hydrogen 0   500

According to another exemplary embodiment of the present disclosure, thedeposited metal film 208 of orthopaedic implant 200 is micro-alloyedaccording to Table 3 below. In this embodiment, the minimum nitrogenconcentration of 488 ppm may ensure that orthopaedic implant 200 hassufficient compressive strength, while the maximum nitrogenconcentration of 1,243 ppm may ensure that orthopaedic implant 200 hassufficient ductility. The balance may include primarily tantalum andother elements such as iron, tungsten, molybdenum, silicon, and nickel,for example.

TABLE 3 Minimum Maximum Concentration Concentration Element (ppm (w/v))(ppm (w/v)) Nitrogen 488 1,243 Oxygen 0 2,000 Hydrogen 0   500

In practice, limiting the oxygen concentration to 1,212 ppm (Table 2)may be more reasonable than limiting the nitrogen concentration to 1,243ppm (Table 3). With reference to FIG. 3, the present inventors believethat a significant portion of any O₂ gas that is introduced intoreaction chamber 304 via air input 314 may react with other processgases in reaction chamber 304, rather than depositing onto poroussubstrate 332. For example, a significant portion of any O₂ gas that isintroduced via air input 314 may react with the H₂ gas that isintroduced via H₂ gas input 312 to form water (H₂O) vapor. Therefore,during the CVD process, oxygen deposition onto porous substrate 332 maybe minimal. Also, after the CVD process, oxygen deposition may beminimized by ensuring that the coated, porous substrate 332 is cooledbefore being removed from reaction chamber 304 and exposed to theatmosphere, because a warm part may undergo more oxidation than a coolpart.

The concentration of oxygen in the deposited metal film 208 may be aslow as 0 ppm (Table 2 and Table 3). Therefore, as indicated in Table 1above, it is within the scope of the present disclosure that the airdelivered via input 314 (FIG. 3) may comprise pure or substantially pureN₂ gas, rather than atmospheric air which contains O₂ gas in addition toN₂ gas. Even when the concentration of oxygen in the deposited metalfilm 208 is as low as 0 ppm, orthopaedic implant 200 may achieve thetargeted compressive strength and the targeted ductility.

The chemical composition of orthopaedic implant 200 may be analyzedusing a suitable chemical determinator to ensure compliance with Table 2or Table 3. An exemplary chemical determinator is the TCH600 SeriesNitrogen/Oxygen/Hydrogen Determinator, which is commercially availablefrom LECO Corportation of St. Joseph, Mich. The chemical determinatormay operate based on fusion in an inert, high-temperature environmentand may include infrared (IR) and thermal conductivity (TC) detectors todetect nitrogen, oxygen, and hydrogen in the material.

Micro-alloying the deposited metal film 208 on orthopaedic implant 200may ensure that orthopaedic implant 200 has a specific compressivestrength (SCS) of at least 24,000 psi, for example. In an exemplaryembodiment of the present disclosure, SCS is determined by subjectingorthopaedic implant 200 to an increasing compressive strain. The appliedcompressive strain may be increased incrementally until, at a maximumcompressive load, 0.04″ of total displacement occurs or compressivefailure occurs, for example. SCS may be calculated by dividing theultimate compressive strength (UCS) of the material by the relativedensity (% RD) of the material, where the UCS equals the maximumcompressive load divided by the cross-sectional area of the material.For example, a material having a relative density of 16% RD and across-sectional area of 0.13 square inches that withstands a maximumcompressive load of 1,300 lbf would have a calculated SCS of 62,500 psi.SCS may be determined using a suitable mechanical testing apparatus,such as the Instron 5567 Universal Testing Instrument, which iscommercially available from Instron of Norwood, Mass.

Also, micro-alloying the deposited metal film 208 on orthopaedic implant200 may ensure that orthopaedic implant 200 has a ductility of at least50%, for example. In an exemplary embodiment of the present disclosure,ductility is determined by subjecting orthopaedic implant 200 to anincreasing compressive strain and measuring the percent reduction incompressive load. If the compressive load decreases by more than 50% ofits maximum value, the material may be deemed too brittle. The ductilityof the material may be determined using a suitable mechanical testingapparatus, such as the above-described Instron 5567 Universal TestingInstrument.

By micro-alloying orthopaedic implant 200 and achieving certain targetedmechanical properties, the material may be densified to a relativedensity less than 18% RD. For example, the material may be densified toa relative density as low as 12% RD, 13% RD, or 14% RD and as high as15% RD, 16% RD, or 17% RD, or within any range delimited by any pair ofthe forgoing values. For purposes of the present disclosure, therelative density of a given piece of material is calculated by dividingthe actual density of the piece of material by the theoretical densityof the deposited metal and multiplying by 100 to express the ratio as apercentage. When the deposited metal is tantalum having a theoreticaldensity of 16.6 g/cm³, the piece of material may have an actual densityless than 2.9 g/cm³ or less than 3.0 g/cm³ (to arrive at less than 18%RD). For example, the piece of material may have an actual density aslow as 2.0 g/cm³ (to arrive at 12% RD), 2.2 g/cm³ (to arrive at 13% RD),or 2.3 g/cm³ (to arrive at 14% RD) and as high as 2.5 g/cm³ (to arriveat 15% RD), 2.7 g/cm³ (to arrive at 16% RD), or 2.8 g/cm³ (to arrive at17% RD). Although the underlying porous substrate and interstitialelements would contribute to the weight of the material, thosecontributions are insignificant and may be ignored such that thematerial is assumed to be entirely metal when calculating the relativedensity.

The ability to reduce the relative density of the material may decreasethe time required to manufacture the material. If the material isrequired to have a relative density of 18% RD, for example, the CVDprocess would continue until the material reaches a relatively hightarget weight. In certain embodiments, 8 cycles, 10 cycles, or 12 cyclesof the CVD process may be required, with each individual cycle lastingmore than 10 hours. However, if the material is able to have a relativedensity of 12% RD, for example, the CVD process may terminate when thematerial reaches a relatively low target weight. In certain embodiments,the CVD process may be shortened by 10 hours, 20 hours, 30 hours, ormore. Such time savings may be recognized while still achieving certaintargeted mechanical properties.

Additionally, the ability to reduce the relative density of the materialmay decrease the inputs and ingredients required to manufacture thematerial. If the material is required to have a relative density of 18%RD, for example, a relatively large amount of tantalum metal would berequired to produce a relatively thick coating on the porous substrate.However, if the material is able to have a relative density of 12% RD,for example, less tantalum metal would be required to produce arelatively thin coating on the porous substrate. Such material savingsmay be recognized while still achieving certain targeted mechanicalproperties.

At this stage, because the material is expected to achieve targetedmechanical properties for implantation, the material is considered to bedensified to a “completed extent.” As used herein, the “completedextent” of densification means that the material need not be furtherdensified or coated before implantation. The material may remainpermanently at the “completed extent” of densification, not justtemporarily between coating cycles, for example. In this respect, the“completed extent” of densification is not an intermediate extent ofdensification between coating cycles. Also, the material may be providedto another party or otherwise prepared for implantation in the“completed extent” without requiring additional coating cycles.

After the material is densified to the “completed extent” during thecoating step 104 of method 100 (FIG. 1), orthopaedic prosthesis 200 maybe subjected to any necessary shaping, processing, sterilizing, orpackaging steps. For example, a polymeric bearing component may besecured onto orthopaedic prosthesis 200 to form an articulating, jointreplacement implant. Exemplary methods for attaching a polymeric bearingcomponent to a highly porous material are described in U.S. PatentApplication Publication No. 2009/0112315 to Fang et al., the entiredisclosure of which is expressly incorporated herein by reference. Asanother example, orthopaedic prosthesis 200 may be coupled to a solidmetal substrate, such as by sintering or diffusion bonding. Exemplarymethods for attaching a highly porous material to a solid metalsubstrate are described in U.S. Pat. No. 7,918,382 to Charlebois et al.and in U.S. Pat. No. 7,686,203 to Rauguth et al., the entire disclosuresof which are expressly incorporated herein by reference.

Finally, in step 106 of method 100 (FIG. 1), orthopaedic prosthesis 200is implanted into a patient's body. The illustrative orthopaedic implant200 of FIG. 2 is hemispherical in shape and is configured to beimplanted into the patient's hip joint as a prosthetic acetabularcomponent. It is also within the scope of the present disclosure thatorthopaedic implant 200 may be a prosthetic proximal femoral componentfor use in the patient's hip joint, a prosthetic distal femoralcomponent for use in the patient's knee joint, a prosthetic tibialcomponent for use in the patient's knee joint, a prosthetic humeralcomponent for use in the patient's shoulder joint, a prosthetic dentalcomponent, or a prosthetic spinal component, for example. Orthopaedicimplant 200 may also be in the shape of a plate, plug, or rod, forexample.

EXAMPLES

The following examples illustrate the impact of micro-alloying a highlyporous tantalum structure.

1. Example 1

a. Design of Experiment

A first experiment was designed and performed to evaluate the SCS of ahighly porous tantalum structure based on two factors: (1) the ratio ofatmospheric air flow rate to chlorine flow rate introduced to the CVDprocess, and (2) the final relative density.

The test samples were RVC cylinders having nominal dimensions of 0.400″in length and 0.400″ in diameter. When coating the samples, theair/chlorine ratio was varied between 0.00 and 0.10, and the finalrelative density of the samples was varied between about 18% RD andabout 22% RD. Other CVD process parameters remained constant throughoutthe experiment, as set forth in Table 4 below.

TABLE 4 CVD Process Parameter Setpoint Chlorine (Cl₂) Gas Flow Rate 900sccm Hydrogen (H₂) Gas Flow Rate 1800 sccm Chlorination ChamberTemperature 500° C. Deposition Chamber Temperature 900° C. VacuumPressure 1.6 Torr Cycle Duration 600 minutes

Each test sample was removed from the CVD apparatus after reaching atarget weight (about 2.4-3.2 grams) corresponding to its specifiedrelative density. Due to the nature of the CVD process, variations of±1% RD, and in certain cases ±1.5% RD, from the specified relativedensities were deemed acceptable.

The samples were subjected to mechanical testing to measure SCS (psi)and were subjected to chemical testing to measure the nitrogenconcentration (ppm) and the oxygen concentration (ppm) in the samples.Such testing methods are described further above.

b. Effect of Relative Density on SCS

Because SCS is effectively normalized for relative density, bydefinition, relative density did not have a statistically significanteffect on SCS. A reduced statistical model was created by removing therelative density factor, as well as the interaction between theair/chlorine ratio factor and the relative density factor.

c. Effect of Air/Chlorine Ratio on SCS

Analysis of the reduced model indicated with high probability (p=0.003)that the air/chlorine ratio accounts for 98.5% of the variation inaverage SCS. Regression analysis of the data resulted in the followingbest-fit (R²=0.8997), exponential relationship between SCS and theair/chlorine ratio:SCS(psi)=18,392e^[12.41(Air Flow Rate(sccm)/Chlorine FlowRate(sccm))]  Equation 1

According to Equation 1 above, strength may be improved by increasingthe air/chlorine ratio during the CVD process. However, increasing theair/chlorine ratio too much could lead to brittle failure. Although noneof the samples in the present study exhibited brittle failure duringcompressive testing, one sample that was manufactured at the highestair/chlorine ratio (0.10) exhibited material separation when subjectedto repeated compressive tests, which may indicate the onset of brittlefailure.

d. Effect of Air/Chlorine Ratio on Nitrogen Concentration

Analysis of the reduced model indicated with high probability (p=0.002)that the air/chlorine ratio accounts for 98.1% of the variation in theaverage nitrogen concentration. Regression analysis of the data resultedin the following best-fit (R²=0.9738), exponential relationship betweenthe air/chlorine ratio and the average nitrogen concentration:Nitrogen Concentration(ppm)=209.88e^[21.748(Air FlowRate(sccm)/ChlorineFlowRate(sccm))]  Equation 2

e. Effect of Air/Chlorine Ratio on Oxygen Concentration

The data indicated that the average oxygen concentration is independentof relative density, but the average oxygen concentration reached amaximum at the center point for relative density (20% RD). Similarly,the data also indicated that the average oxygen concentration isindependent of the air/chlorine ratio, but the average oxygenconcentration reached a maximum at the center point for the air/chlorineratio (0.05).

Neither the air/chlorine ratio, the relative density, nor theinteraction between the air/chlorine ratio and the relative density hada statistically significant effect on the standard deviation of theoxygen concentration. Regression analysis of the data indicated nosignificant statistical relationship (R²=0.0012) between theair/chlorine ratio and the average oxygen concentration.

N₂ and O₂ gases are both introduced proportionally into the CVD reactionchamber in the incoming atmospheric air stream, so the present inventorsoriginally anticipated that the relationship between the air/chlorineratio and the average oxygen concentration in the samples would besimilar to the relationship between the air/chlorine ratio and theaverage nitrogen concentration in the samples (Equation 2). The presentinventors now believe, however, that a significant portion of theintroduced O₂ gas reacts with other process gases in the chamber, ratherthan depositing onto the porous substrate. For example, the introducedO₂ gas may react with the introduced H₂ gas to form water (H₂O) vapor.

f. Effect of Nitrogen Concentration on SCS

Given the high correlation between the air/chlorine ratio and SCS(Equation 1) and the high correlation between the air/chlorine ratio andthe average nitrogen concentration (Equation 2), the inventorsanticipated a high correlation between SCS and the average nitrogenconcentration. Regression analysis of the data resulted in the followingbest-fit (R²=0.9697), linear relationship between SCS and the averagenitrogen concentration:SCS(psi)=10,309+33.681*(Nitrogen Concentration(ppm))  Equation 3

According to Equation 3 above, micro-alloying a highly porous tantalummaterial with nitrogen is a potential mechanism for increasing SCS.

Although regression analysis indicated a high correlation between SCSand the average nitrogen concentration (Equation 3), regression analysisdid not indicate a statistically significant correlation (R²=0.0469)between SCS and the average oxygen concentration.

2. Example 2

a. Design of Experiment

A second experiment was designed and performed to evaluate the SCS andthe ductility of a highly porous tantalum structure based on theconcentration of nitrogen and oxygen in the structure.

The test samples were RVC cylinders having nominal dimensions of 0.400″in length and 0.400″ in diameter. A two-step CVD process was performedaccording to Table 5 below to produce coated samples having nitrogenconcentrations between about 350 ppm and about 1,200 ppm, oxygenconcentrations between about 300 ppm and about 1,200 ppm, and relativedensities between about 12% RD and about 18% RD.

TABLE 5 CVD Process Parameter Operating Range Step 1 Chlorine (Cl₂) GasFlow Rate 600-984 sccm Hydrogen (H₂) Gas Flow Rate 1,150-2,200 sccmAtmospheric Air Flow Rate 10-40 sccm Chlorination Chamber Temperature500° C. Deposition Chamber Temperature 900-970° C. Vacuum Pressure 1.6Torr Total Duration 5,500-7,500 minutes Step 2 Chlorine (Cl₂) Gas FlowRate 0 sccm Hydrogen (H₂) Gas Flow Rate 0 sccm Atmospheric Air Flow Rate15-45 sccm Deposition Chamber Temperature 485-515° C. Vacuum Pressure1.0 Torr Total Duration 120-150 minutes

The samples were subjected to mechanical testing to measure SCS (psi)and ductility (%) and were subjected to chemical testing to measure thenitrogen concentration (ppm), the oxygen concentration (ppm), and thehydrogen concentration (ppm) in the samples. Such testing methods aredescribed further above.

b. Correlation between Nitrogen and Oxygen Concentrations

Analysis of the data indicated no statistically significationcorrelation (p=0.298, a=0.05) between nitrogen and oxygen concentrationsin the samples. Thus, the effects of nitrogen and oxygen concentrationsmay be evaluated separately.

c. Effect of Relative Density on Nitrogen and Oxygen Concentrations

Analysis of the data indicated no statistically significationcorrelation between relative density and the nitrogen concentrations inthe samples (p=0.186, a=0.05) or between relative density and the oxygenconcentrations in the samples (p=0.303, a=0.05). Thus, relative densitymay be discounted when analyzing the effects of nitrogen and oxygenconcentrations.

d. Effect of Nitrogen and Oxygen Concentrations on SCS

Regression analysis of the data resulted in the following best-fit(R²=0.861), linear relationship between SCS and the average nitrogen andoxygen concentrations:SCS(psi)=11,361+38.3*(Nitrogen Concentration(ppm))+11.2*(OxygenConcentration(ppm))  Equation 4

The entire Equation 4 was found to be statistically significant(p=0.000, a=0.05). Also, each individual term within Equation 4—theconstant term (p=0.001, a=0.05), the nitrogen concentration term(p=0.000, a=0.05), and the oxygen concentration term (p=0.012,a=0.05)—was found to be statistically significant.

According to Equation 4, increasing the concentration of nitrogen and/oroxygen increases SCS because both signs are positive. Also, the nitrogenconcentration has a larger effect on SCS than the oxygen concentrationbecause the nitrogen concentration term is larger in magnitude than theoxygen concentration term. A certain minimum nitrogen concentration or acertain minimum oxygen concentration may ensure SCS above the specifiedminimum of 24,000 psi, for example.

Regression analysis of the data resulted in the following best-fit(R²=0.147), linear relationship between SCS and the average oxygenconcentration alone:SCS(psi)=33,973+21.27*(Oxygen Concentration(ppm))  Equation 5

Also, regression analysis of the data resulted in the following best-fit(R²=0.832), linear relationship between SCS and the average nitrogenconcentration alone:SCS(psi)=16,308+40.17*(Nitrogen Concentration(ppm))  Equation 6

Equations 5 and 6 are plotted in FIGS. 4A and 4B, respectively, alongwith 95% prediction intervals and 95% confidence intervals. For anychosen oxygen concentration value along the x-axis of FIG. 4A, thevertical distance between the prediction interval lines represents theeffect of varying the nitrogen concentration while holding the oxygenconcentration constant. Similarly, for any chosen nitrogen concentrationvalue along the x-axis of FIG. 4B, the vertical distance between theprediction interval lines represents the effect of varying the oxygenconcentration while holding the nitrogen concentration constant.

With respect to FIG. 4A, the lower 95% prediction interval for oxygen isconsistently below the specified minimum SCS of 24,000 psi. Thus, nominimum oxygen concentration within the tested range of about 300 ppmand about 1,200 ppm would ensure SCS of at least 24,000 psi with 95%confidence.

With respect to FIG. 4B, the lower 95% prediction interval for nitrogenintersects the 24,000 psi reference line at a nitrogen concentration of488 ppm (see circled intersection point in FIG. 4B). Thus, even in theabsence of oxygen, a nitrogen concentration of at least 488 ppm wouldensure SCS of at least 24,000 psi with 95% confidence.

e. Effect of Nitrogen and Oxygen Concentrations on Ductility

Regression analysis of the data resulted in the following best-fit(R²=0.505), linear relationship between ductility and the averagenitrogen and oxygen concentrations:Ductility(%)=1.09−0.000203*(NitrogenConcentration(ppm))−0.000150*(Oxygen Concentration ppm))  Equation 7

The entire Equation 7 was found to be statistically significant(p=0.000, a=0.05). Also, each individual term within Equation 7—theconstant term (p=0.000, a=0.05), the nitrogen concentration term(p=0.000, a=0.05), and the oxygen concentration term (p=0.022,a=0.05)—was found to be statistically significant.

According to Equation 7, increasing the concentration of nitrogen and/oroxygen decreases ductility because both signs are negative. A certainmaximum nitrogen concentration or a certain maximum oxygen concentrationmay ensure ductility above the specified minimum of 50%, for example.

Regression analysis of the data resulted in the following best-fit(R²=0.217), linear relationship between ductility and the average oxygenconcentration alone:Ductility(%)=0.9721−0.000204*(Oxygen Concentration(ppm))  Equation 8

Also, regression analysis of the data resulted in the following best-fit(R²=0.430), linear relationship between ductility and the averagenitrogen concentration alone:Ductility(%)=1.026−0.000228*(Nitrogen Concentration(ppm))  Equation 9

Equations 8 and 9 are plotted in FIGS. 5A and 5B, respectively, alongwith 95% prediction intervals and 95% confidence intervals. For anychosen oxygen concentration value along the x-axis of FIG. 5A, thevertical distance between the prediction interval lines represents theeffect of varying the nitrogen concentration while holding the oxygenconcentration constant. Similarly, for any chosen nitrogen concentrationvalue along the x-axis of FIG. 5B, the vertical distance between theprediction interval lines represents the effect of varying the oxygenconcentration while holding the nitrogen concentration constant.

With respect to FIG. 5A, the lower 95% prediction interval for oxygenintersects the 50% reference line at an oxygen concentration of 1,212ppm (see circled intersection point in FIG. 5A). Thus, even in thepresence of maximum nitrogen, an upper limit oxygen concentration of1,212 ppm would ensure ductility of at least 50% with 95% confidence.

With respect to FIG. 5B, the lower 95% prediction interval for nitrogenintersects the 50% reference line at a nitrogen concentration of 1,243ppm (see circled intersection point in FIG. 5B). Thus, even in thepresence of maximum oxygen, an upper limit nitrogen concentration of1,243 ppm would ensure ductility of at least 50% with 95% confidence.

While this invention has been described as having exemplary designs, thepresent invention can be further modified within the spirit and scope ofthis disclosure. This application is therefore intended to cover anyvariations, uses, or adaptations of the invention using its generalprinciples. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

What is claimed is:
 1. A highly porous biomaterial configured to beimplanted in a patient's body, the highly porous biomaterial comprising:a porous substrate having a plurality of ligaments that define pores ofthe porous substrate; and a biocompatible metal coating applied to theplurality of ligaments of the porous substrate, the biocompatible metalof the coating having a minimum nitrogen concentration of 488 ppm, thehighly porous biomaterial having a relative density less than 18% and aspecific compressive strength of at least 24,000 psi, the relativedensity being a percentage obtained by dividing an actual density of thehighly porous biomaterial by a theoretical density of the biocompatiblemetal of the coating.
 2. The highly porous biomaterial of claim 1,wherein the highly porous biomaterial has a relative density of at least12% and less than 18%.
 3. The highly porous biomaterial of claim 1,wherein the highly porous biomaterial has a ductility of at least 50%.4. The highly porous biomaterial of claim 1, wherein the poroussubstrate comprises reticulated vitreous carbon.
 5. The highly porousbiomaterial of claim 1, wherein the biocompatible metal of the coatingcomprises tantalum, the theoretical density of tantalum being 16.6g/cm³.
 6. The highly porous biomaterial of claim 5, wherein the actualdensity of the highly porous biomaterial is 3.0 g/cm³ or less.
 7. Thehighly porous biomaterial of claim 1, wherein the biocompatible metal ofthe coating has a minimum oxygen concentration of 0 ppm.
 8. The highlyporous biomaterial of claim 1, wherein the biocompatible metal of thecoating has a maximum nitrogen concentration of 1,243 ppm.
 9. The highlyporous biomaterial of claim 8, wherein the biocompatible metal of thecoating has a maximum oxygen concentration of 2,000 ppm.
 10. The highlyporous biomaterial of claim 1, wherein the biocompatible metal of thecoating has a maximum oxygen concentration of 1,212 ppm.
 11. The highlyporous biomaterial of claim 10, wherein the biocompatible metal of thecoating has a maximum nitrogen concentration of 2,200 ppm.
 12. Thehighly porous biomaterial of claim 1, wherein the biocompatible metal ofthe coating has a maximum hydrogen concentration of 500 ppm.
 13. Anorthopaedic implant made at least in part of the highly porousbiomaterial of claim
 1. 14. The highly porous biomaterial of claim 1,wherein the highly porous biomaterial is formed by a method ofmanufacturing comprising: providing the porous substrate having theplurality of ligaments that define the pores of the porous substrate;depositing the biocompatible metal coating onto the plurality ofligaments of the porous substrate; and setting at least one of: amaximum oxygen concentration in the metal coating at 1,212 ppm; and amaximum nitrogen concentration in the metal coating at 1,243 ppm. 15.The highly porous biomaterial of claim 14, wherein the setting providesthe highly porous biomaterial with a specific compressive strength of atleast 24,000 psi.
 16. The highly porous biomaterial of claim 14, whereinthe method further comprises setting a minimum nitrogen concentration inthe metal coating at 488 ppm.
 17. The highly porous biomaterial of claim1, wherein the highly porous biomaterial is formed by a method ofmanufacturing comprising: providing the porous substrate having theplurality of ligaments that define the pores of the porous substrate;depositing the biocompatible metal coating onto the plurality ofligaments of the porous substrate; and setting a minimum nitrogenconcentration in the metal coating at 488 ppm.
 18. The highly porousbiomaterial of claim 17, wherein the setting provides the highly porousbiomaterial with a ductility of at least 50%.
 19. The highly porousbiomaterial of claim 17, wherein the method further comprises setting atleast one of: a maximum oxygen concentration in the metal coating at1,212 ppm; and a maximum nitrogen concentration in the metal coating at1,243 ppm.
 20. The highly porous biomaterial of claim 1, wherein thehighly porous biomaterial is formed by a method of manufacturingcomprising: providing the porous substrate having the plurality ofligaments that define the pores of the porous substrate; and depositingthe biocompatible metal coating onto the plurality of ligaments of theporous substrate to a completed extent, the highly porous biomaterialhaving a relative density less than 18% at the completed extent, therelative density being a percentage obtained by dividing an actualdensity of the highly porous biomaterial by a theoretical density of thebiocompatible metal of the coating.
 21. The highly porous biomaterial ofclaim 20, wherein the method further comprises providing the highlyporous biomaterial at the completed extent for implantation in apatient's body without depositing more of the biocompatible metalcoating onto the plurality of ligaments.
 22. The highly porousbiomaterial of claim 20, wherein the highly porous biomaterial has arelative density of at least 12% and less than 18% at the completedextent.